A number of people, typically elderly, suffer from chronic respiratory insufficiency due to restrictive airway disease, obstructive pulmonary disease, neuromuscular disorders or other complications. Symptoms of chronic respiratory insufficiency include shortness of breath, weight loss, headaches and sleeplessness.
To alleviate these symptoms, physicians often prescribe to these patients use of concentrated oxygen, i.e., an oxygen enriched gas. While some patients have their concentrated oxygen delivered to them as liquid oxygen or in high pressure oxygen cylinders, a great number of the patients use a small portable commercially available oxygen concentrator to obtain their supply of concentrated oxygen. Exemplary portable oxygen concentrator include Respironics Millenium Model 605 Oxygen Concentrator (Resperonics, Kennesaw, Ga.), AirSep NewLife Oxygen Concentrator (AirSep Corp., Buffalo, N.Y.), Puritan-Bennett Aeris 590 model Oxygen Concentrator, (Puritan-Bennett Corp., Pleasanton, Calif.), as well as those disclosed in U.S. Pat. No. 5,893,275, which is incorporated herein by reference in its entirety. Some patients use these portable oxygen concentrators in combination with an oxygen liquefaction device to produce liquid oxygen.
Liquid oxygen can be stored and transferred to other portable vessels, thereby allowing the patients a freedom of movement without having to be near the oxygen concentrator. One of the major problems with a conventional portable gas liquefaction device is the accumulation frost (i.e., rime) within the cryogenic unit of the device. Frost formation on the cryogenic unit forms an insulating barrier, which reduces the efficiency of the cryogenic unit, and hence the liquefaction rate of the oxygen enriched gas. In addition, a severe frost accumulation can cause partial or full blockage of the gaseous or liquefied oxygen flow through the device, thus potentially creating a dangerous situation. To prevent blockage, the cryogenic unit must be powered down and defrosted at regular intervals. Typically, the cryogenic unit is defrosted at least once a month. However, the actual frequency of defrosting the cryogenic unit depends on a variety of factors, such as the amount of moisture in the oxygen enriched gas, the amount of moisture in the transfill gas, the humidity and temperature at which the cryogenic device is used, as well as other factors that effect the frost formation. Powering down the cryogenic unit obviously has disadvantages, such as inability to use the unit as well as generally requiring periodic, expensive deliveries of supplemental liquid oxygen during the time it is being defrosted.
Therefore, there is a need for a device that reduces the frequency of cryogenic unit power downs.